MAX26239/MAX26240

4.5V to 36V, 6A Buck-Boost Converters

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Detailed Description

The MAX26239/MAX26240 are small, synchronous buck-boost converters with integrated high-side and low-side

switches. The ICs are designed to deliver up to 6.0A with input voltages from +4.5.0V to +36V while using only 95μA

quiescent current at no load. The MAX26239/MAX26240 provide an accurate output voltage of ±2% within the normal

operation input range. Voltage quality can be monitored by observing the PGOOD signal.

The MAX26239/MAX26240 offer fixed output voltages and programmable output voltages in the range of 3V to 20V.

Frequency is fixed with 400kHz, and 2.1MHz options. The 2.1MHz frequency allows for small external components and

reduced output ripple, and guarantees no AM interference. The IC automatically enters skip mode at light loads with a

low quiescent current of 95μA at no load. It can operate with ±6% spread-spectrum frequency modulation designed to

minimize EMI radiated emissions.

H-Bridge Operation

The MAX26239/MAX26240 H-bridge configuration is shown in the Simplified Block Diagram. The H-bridge consists of

the four switches: HS1, LS1, HS2, and LS2. Switches HS1 and LS1 are in series with the input voltage, and switches

HS2 and LS2 are connected to the output. The inductor is connected between LX1 and LX2. There are three operation

modes depending on the ratio of the input and output voltage: buck mode, boost mode, and buck-boost mode.

Buck Mode

When the input voltage is much higher than the output voltage, the MAX26239/MAX26240 operate in pure buck mode. In

this mode, switch HS2 is always on and switch LS2 is always off, while the switches HS1 and LS1 switch at the switching

frequency. The IC uses an peak-current-mode control scheme to determine the ON pulse width for the switches HS1 and

LS1. The switches HS1 and LS1 will alternate, behaving like a synchronous buck converter.

Boost Mode

When the input voltage is much lower than the output voltage, the MAX26239/MAX26240 operate in pure boost mode. In

this boost configuration, the switch HS1 is always on and the switch LS1 is always off, while the switches HS2 and LS2

are operating at the switching frequency. The MAX26240 uses a peak-current-mode control scheme to determine the ON

pulse width for the switches HS2 and LS2. The switches HS2 and LS2 switch as a synchronous boost converter.

Buck-Boost Mode

With the input voltage close to the output voltage, the MAX26239/MAX26240 operate in buck-boost mode. During the

buck-boost transition region, all four switches are turned on/off at the switching frequency as needed to maintain high

efficiency and regulated output voltage in the transition region.

Linear Regulator Output (VCC)

The devices include a 1.8V linear regulator (VCC) that provides power to the internal circuit blocks. Connect a 4.7μF (min)

ceramic capacitor from VCC to GND. During startup, the bias regulator draws power from the input and switches over to

the output after the startup is complete. For output voltages less than 1.8V, the bias regulator is always supplied from the

input.

Soft-Start

The MAX26239/MAX26240 include a 2.5ms soft-start time. Soft-start time limits startup inrush current by forcing the

output voltage to ramp up towards its regulation point. The soft-start ramp rate is set at 2.5ms.

Current-Limit/Hiccup Mode

The devices feature a current limit that protects the device against short-circuit and overload conditions at the output. In

the event of a short-circuit or overload condition, the high-side MOSFET remains on until the inductor current reaches the

high-side MOSFET’s current-limit threshold. The converter then turns on the low-side MOSFET to allow the inductor

current to ramp down. Once the inductor current crosses below the low-side MOSFET current-limit threshold, the

converter turns on the high-side MOSFET again. This cycle repeats until the short-circuit or overload condition is removed.

If the device reaches the current limit with an output voltage below 50% of the target, hiccup mode is enabled and the

output turns off for 5ms, then the IC attempts to power up through soft-start again.

Power-Good Output (PGOOD)

The devices feature an open-drain, power-good indicator (PGOOD). PGOOD asserts low when the output voltage drops

below the 93% (typ) falling threshold. PGOOD deasserts when the output voltage rises above the 94% (typ) rising

threshold. Connect PGOOD to the output or external I/O voltage with a pull-up resistor.